Methods are described for the spinning of carbon nanotube fibers containing a high loading of nanotubes in a fiber component; the stabilization of spun materials by post-spinning processes; and the application of such materials made by spinning.
Since the discovery of carbon nanotubes by Iijima and coworkers (Nature 354, 56-58, (1991) and Nature 361, 603-605 (1993)) various types of carbon nanotubes (NTs) have been synthesized. A single-wall carbon nanotube (SWNT) consists of a single layer of graphite that has been wound into a seamless tube having a nanoscale diameter. A multi-wall carbon nanotube (MWNT), on the other hand, comprises two or more such cylindrical graphite layers that are coaxial. Both single-wall and multi-wall nanotubes have been obtained using various synthetic routes, which typically involve the use of metallic catalysts and very high processing temperatures. Typical synthesis routes are those employing a carbon arc, laser evaporation of carbon targets, and chemical vapor deposition (CVD).
SWNTs are produced by the carbon-arc discharge technique using a pure carbon cathode and a carbon anode containing a mixture of graphite powder and catalytic metal(s), like Fe, Ni, Co and Cu (D. S. Bethune et al. Nature 363, 605-7 (1993) and S. Iijima and T. Ichihashi, Nature 363, 603-5 (1993)). C. Journet et al. (Nature 388, 756-758 (1997)) have described an improved carbon-arc method for the synthesis of SWNTs which uses Ni/Y (4.2/1 atom %) as the catalyst. Co-vaporization of carbon and the metal catalyst in the arc generator produced a web-like deposit of SWNTs that is intimately mixed with fullerene-containing soot.
Smalley""s group (A. Thess et al., Science 273, 483-487(1996)) developed a pulsed laser vaporization technique for synthesis of SWNT bundles from a carbon target containing 1 to 2% (w/w) Ni/Co. The dual laser synthesis, purification and processing of single-wall nanotubes has been described in the following references: J. Liu et al., Science 280, 1253 (1998); A. G. Rinzler et al., Applied Physics A 67, 29 (1998); A. G. Rinzler et al., Science 269, 1550 (1995); and H. Dai, et al., Nature 384, 147 (1996).
The CVD method described by Cheng et al. (Appl. Phys. Lett. 72, 3282 (1998)) involves the pyrolysis of a mixture of benzene with 1 to 5% thiophene or methane, using ferrocene as a floating catalyst and 10% hydrogen in argon as the carrier gas. The nanotubes form in the reaction zone of a cylindrical furnace held at 1100-1200xc2x0 C. Depending on the thiophene concentration, the nanotubes form as either multi-wall nanotubes or bundles of single-wall nanotubes. Another useful method for growing single-wall nanotubes uses methane as the precursor, ferric nitrate contained on an alumina catalyst bed, and a reaction temperature of 1000xc2x0 C.
Another CVD synthesis process was described by R. E. Smalley et al. in PCT International Application No. WO 99-US25702, WO 99-US21367 and by P. Nikolaev et al. in Chem. Phys. Lett. 313, 91-97 (1999). This process, known as the HiPco process, utilizes high pressure (typically 10-100 atm) carbon monoxide gas as the carbon source, and nanometer sized metal particles (formed in-situ within the gas stream from organo-metallic precursors) to catalyze the growth of single-wall carbon nanotubes. The preferred catalyst precursors are iron carbonyl (Fe(CO)5) and mixtures of iron carbonyl and nickel carbonyl (Ni(CO)4). The HiPco process produces a SWNT product that is essentially free of carbonaceous impurities, which are the major component of the laser-evaporation and carbon-arc products. The process enables control of the range of nanotube diameters produced, by controlling the nucleation and size of the metal cluster catalyst particles. In this way, it is possible to produce unusually small nanotube diameters, about 0.6 to 0.9 nm.
The nanotube-containing products of the laser-evaporation and carbon-arc processes invariably contain a variety of carbonaceous impurities, including various fullerenes and less ordered forms of carbon soot. The carbonaceous impurity content in the raw products of the laser and carbon arc processes typically exceeds 50 weight %. Purification of these products generally relies on selective dissolution of the catalyst metals and highly ordered carbon clusters (called fullerenes) followed by selective oxidation of the less ordered carbonaceous impurities. A typical purification process is described by Lui et al. in Science 280, 1253 (1998). This method involves refluxing the crude product in 2.6 M nitric acid for 45 hours, suspending the nanotubes in pH 10 NaOH aqueous solution using a surfactant (e.g, Triton X-100 from Aldrich, Milwaukee, Wis.), followed by filtration using a cross-flow filtration system. While the effects of these purification processes on the nanotubes themselves are not completely understood, it is believed that the nanotubes are shortened by oxidation.
As discussed by B. I. Jakobson and R. E. Smalley (American Scientist 85, 325, 1997) SWNT and MWNT materials are promising for a wide variety of potential applications because of the exceptional physical and chemical properties exhibited by the individual nanotubes or nanotube bundles. Some SWNT properties of particular relevance include metallic and semiconducting electrical conductivity, depending on the specific molecular structure, extensional elastic modulus of 0.6 TPa or higher, tensile strengths on the order of ten to one hundred GPa, and surface areas that can exceed 300 m2/g.
The proposed applications of carbon nanotubes include mechanical applications, such as in high-strength composites, electrical applications, and multifunctional applications in which different properties aspects of the carbon nanotubes are simultaneously utilized. Tennent et al. in U.S. Pat. No. 6,031,711 describe the application of sheets of carbon nanotubes as high performance supercapacitors. In this application, a voltage difference is applied to two high-surface-area carbon nanotube electrodes that are immersed in a solid or liquid electrolyte. Current flows in the charging circuit, thereby injecting charge in the nanotubes, by creating an electrostatic double layer near the nanotube surfaces.
The application of carbon nanotube sheets as electromechanical actuators has been recently described (R. H. Baughman et al., Science 284, 1340 (1999)). These actuators utilize dimension changes that result from the double-layer electrochemical charge injection into high-surface-area carbon nanotube electrodes. If carbon nanotubes can be assembled into high modulus and high strength assemblies (such as filaments, ribbons, yarns, or sheets) that maintain their ability to electrochemically store charge, then superior actuator performance should be obtainable. The problem has been that no methods are presently available for the manufacture of nanotube articles that have these needed characteristics.
These and other promising applications require assembling the individual nanotubes into macroscopic arrays that effectively use the attractive properties of the individual nanotubes. This obstacle has so far hindered applications development. The problem is that MWNTs and SWNTs are insoluble in ordinary aqueous solvents and do not form melts even at very high temperatures. Under certain conditions, and with the aid of surfactants and ultrasonic dispersion, bundles of SWNTs can be made to form a stable colloidal suspension in an aqueous medium. Filtration of these suspensions on a fine-pore filter medium, as described by Lui et al. in Science 280, 1253 (1998), results in the production of a paper-like sheet which has been called xe2x80x9cbucky paperxe2x80x9d (in reference to buckminsterfullerene, or C60, the first member of the fullerene family of carbon cluster molecules). Such sheets, which can range in conveniently obtainable thickness from 10-100 microns, possess mechanical strength derived from the micro-scale entanglement of the nanotube bundles. Bucky paper preserves the large accessible surface area of the nanotube bundles, but typically exhibit greatly reduced elastic modulus values (a few GPa) that are a very small fraction of the intrinsic elastic modulus of either the individual SWNTs or the SWNT bundles.
A recently reported method for processing carbon nanotubes provides nanotube fibers whose mechanical properties significantly surpass those of ordinary bucky paper. This method was described by B. Vigolo et al. in Science 290, 1331 (2000) and by R. H. Baughman in Science 290, 1310 (2000). According to this process, the carbon nanotubes are first dispersed in an aqueous or non-aqueous solvent with the aid of a surfactant. A narrow jet of this nanotube dispersion is then injected into a rotating bath of a more viscous liquid in such a way that shear forces at the point of injection cause partial aggregation and alignment of the dispersed nanotube bundles. This viscous liquid contains an agent or agents, which act to neutralize the dispersing action of the surfactant. Consequently, the jet of dispersed nanotubes is rapidly coagulated into a low-density array of entangled nanotubesxe2x80x94thereby gaining a small (but useful) amount of tensile strength. The wet filament is then washed in water, and the washed filament is subsequently withdrawn from the wash bath and dried. During which draw-dry process, capillary forces collapse the loosely tangled array of nanotubes into a compact thin fiber having a density of about 1.5 gm/cc (close to the theoretical density of a compact array of carbon nanotubes). This total process will henceforth be referred to as the coagulation spinning (CS) process.
In a typical CS process, as described by Bernier et al., the nanotubes are dispersed in water with the aid of sodium dodecyl sulphate (SDS) surfactant. The viscous carrier liquid is an aqueous solution of polyvinyl alcohol (PVA) in which the PVA also serves to neutralize the effect of the SDS surfactant by directly replacing these molecules on the NT surfaces. Bernier et al. describe preferred concentrations for the various ingredients, and viscosity ranges and flow velocities of the spinning solutions. Polarized light microscopy of the coagulated nanotube fibers confirms preferential alignment of the NTs along the fiber axis. Further evidence of NT alignment is provided by the measured extensional elastic modulus, which is approximately 10 GPa for the final CS fibers, as compared to typically 1 GPa for bucky paper.
Unfortunately, the fibers made by the CS process are not useful in applications as electrodes immersed in liquid electrolytes because of a surprising shape memory effect. This shape memory effect causes the CS fibers to dramatically swell (by 100% or more) and lose most of their dry-state modulus and strength. Because of this structural instability of fibers made by the CS process, they are unusable for critically important applications that use liquid electrolytes, such as in supercapacitors and in electromechanical actuators. In contrast, as-produced bucky paper made from the same nanotubes can be used for both capacitor and actuator devices that use liquid electrolytes.
Yashi et al. (Y. K. Yashi et al., Materials Research Society Symposium Proceedings, xe2x80x9cScience and Technology of Fullerene Materials,xe2x80x9d 359, pg. 81-6, 1995) attempt to overcome the problems inherent in the CS process by using a method for forming a bundle of aligned carbon nanotubes by extruding a mixture of carbon nanotubes and polypropylene through a small die having a diameter of 2 mm which is maintained at 200xc2x0 C. These materials do not have properties that would render them useful in actual mechanical or electrochemical applications, since they are made with such a low loading of the carbon nanotubes and the polypropylene remains in the final product. In the present method, the polymer carrier is removed.
Glatkowski et al. (U.S. Pat. No. 6,265,466) teach a method for preparing an electromagnetic shielding composite having nanotubes wherein the nanotubes are oriented when a shearing force is applied to the composite. The method includes a step of providing a polymer with an amount of nanotubes, and imparting a shearing force to the polymer and nanotubes to orient the nanotubes. Glatkowski et al. generically teach that the nanotube concentration can be as high as 15 wt %, but it is preferable that the concentration is 0.1 to 1.5 wt %. These materials do not have properties that would render them useful in actual mechanical or electrochemical applications, since they are made with such a low loading of the carbon nanotubes. In the present method, a much higher shearing force is used to orient the nanotubes, thereby allowing for a much higher concentration of the nanotubes in the final product.
Lobovsky et al. (copending U.S. Ser. No. 09/946432) teach a method of forming fibers, ribbons and yarns, wherein the carbon nanotubes are first dispersed in an aqueous or non-aqueous solvent with the aid of a surfactant in the CS process described above. The present method has the advantage over the method of Lobovsky et al. in that the present method provides more freedom in the shaping of the final product and it allows for increased loadings of the carbon nanotubes, and as such can be used in a greater variety of applications.
Thus, the prior art processes fail to provide materials containing carbon nanotube fibers that have high modulus, high strength, and high toughness for structural applications, as well as for multifunctional applications where extreme mechanical properties are combined with other functionalities, such as actuation, mechanical energy harvesting, mechanical dampening, thermal energy harvesting, and energy storage. Although the individual nanotube fibers have very attractive performance, the prior art has not demonstrated processes where these properties of the individual nanotubes can be effectively used in macrofibers composed of the nanofibers.
The present invention solves the problems associated with prior art processes, by providing an efficient method for melt spinning carbon nanotube composites wherein the carbon nanotubes have a high loading in the composites (greater than 8%), and such high loadings result in the mechanical properties, electronic conductivity, or electrochemical activity needed for key applications.
The present invention, in part, is drawn to a process which enables the spinning of carbon nanotube fibers containing a high loading of nanotubes in a fibrous component or a layer. This spinning process is enabled by the use of one or more fiber components as a support and alignment element for another fiber component that contains a polymer that is heavily loaded with carbon nanotubes. This process, which is generically applicable to various types of carbonaceous and non-carbonaceous nanofibers, provides nanotube-containing fibers having complex cross-sections that are particularly applicable for mechanical reinforcement, thermal and electrical conduction, energy storage, energy harvesting, and electromechanical actuation.
The process comprises a step of melt flowing a first composition comprising a core polymer thereby aligning the molecules of the core polymer to be in a substantially parallel arrangement in the direction of the flow and combining a second composition comprising a sheath polymer in the melt phase and carbon nanotubes such that the carbon nanotubes align in the general direction of the core polymer molecules, and a step of removing at least 50 wt % of the core and sheath polymer molecules.
The present invention, in part, is drawn to a structure comprising carbon nanotubes wherein the carbon nanotubes are oriented lengthwise in the same general direction and in a high density.
The present invention, in part, is drawn to an apparatus for preparing the structure comprising aligned carbon nanotubes, said apparatus comprising an upper spinneret and a lower spinneret arranged in such a manner as to create a high shear force on a mixture of polymers and carbon nanotubes to align the carbon nanotubes in the general direction of the flow.
Advantages of the present invention will become more apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.